When using white LEDs as a light source, the (ηLED) or wall efficiency is typically 20% to 30% of input power. This is the effective percentage of electricity that is converted to visible light, while the remaining 70% to 80% of input power is converted to heat that must be conducted from the LED die to the underlying heat sink, housing and finally to the surrounding air to maintain an acceptable LED junction temperature. In the United States, the goals of solid state lighting have been identified by the Department of Energy (DOE) and the Optoelectronics Industry Development Association (OIDA). The DOE would like to reproduce the spectrum of sunlight at 50% system efficiency, while the OIDA goal is to achieve an efficiency of 200 lm/W with good color rendering by 2020.
There is a feverish race to achieve LED and/or HBLED modules that can cannibalize the market for incandescent bulbs in the area of illumination. In order to achieve such a goal in moving LED's from indication applications to general illumination thermal management has become a major area in need of innovation. The thermal resistance of LED packages together with the maximum operating temperature determines the maximum power that can be dissipated in the package. At the outset of LED packaging in the 1960's the thermal resistance of a 5 mm package (still used for low power LEDs) typically would be 250 K/W.
Some LED packages utilize surface mount technology (SMT) as illustrated in the example of FIG. 1. The key feature of SMT technology, is the onboard heat sink typically made of copper or aluminum. SMT packaging approaches can generally typically achieve 6-12 K/W thermal resistance. Some SMT packaging having a thermal resistance of 2.5 K/W and 9 W of power handling. This level of thermal resistance is typically achieved when the junction temperature reaches approximately 432K (159° C.).
Note that the LED junction temperature is directly related to the emission spectrum and the spectrum shift caused by slight variations in temperature is generally sensed by the human eye. Thus, considerable effort should be made in maintaining the junction temperature substantially constant and low to ensure robust and reliable operations of LEDs for various applications. In addition, packaging multiple LED dies in a single SMT package is a difficult task due to thermal crosstalk. Thus an incandescent bulb replacement for general illumination is currently out of reach of the majority of LED vendors.